Selecting an Air Compressor

First and foremost, the compressor must be big enough to do the job of handling peak loads without undue strain and waiting for the compressor to build up to the required pressure. Select the compressor that will do the job and allow time between pumping cycles to permit ample cooling. Do not buy a compressor that is too small!

The following series of charts will be helpful in selecting the proper size air compressor for the job and are important in checking and installing the compressor. If you have additional questions as to which air compressor is most appropriate for your needs, please feel free to email J-Air at support@jaircompressors.com.

---

Step #1: Single-Stage vs. Two-Stage Compressor
Single stage compressors take air into their cylinders at atmospheric pressure and compress it with a single stroke of the piston directly to the desired pressure. The air is then passed through a cooler, called an aftercooler, which removes some of the heat of compression. This type of compressor is most commonly used for applications where the pressure requirements are not in excess of 150 psi.

Efficiency varies with pressure, being lower at higher pressures. At high pressure or above 130 psi the efficiency of a single stage compressor falls off rapidly and a two-stage compressor should be used.

Note that single stage efficiency (55% to 70%) is lower than two-stage (75% to 80%).

Step #2: Control Description
Automatic Operation : In automatic (or stop-start) operation, the compressor stops when the pressure in the storage tank reaches the pre-determined maximum pressure (called the cut-out pressure) and starts when the pressure in the tank falls to a pre-determined minimum pressure (called the cut-in pressure).

Continuous Operation (or constant running control) : The compressor continues to run without pumping air after the cut-out pressure has been reached. This is accomplished through either a suction valve unloader, which opens the suction valve or a discharge unloader valve and prevents pumping during compression. This type of control is most widely used where the requirements for air are constant for a long period of time, or if automatic control would require motor starts more than 12 times per hour.

Dual Operation : The dual operation is a combination of automatic and continuous controls in conjunction with a method of changing from one control to another. This option is available only on electric models, for situations such as limiting high start-up amps on the motor, or applications requiring frequent starts/stops.

Step #3: Approximate Cubic Feet per Minute (CFM) Required to Operate Various Pneumatic Equipment
CFM free air is a volume of air at a given pressure expressed as its volume at intake conditions (normally 14.7 psi ABS. and 70° F). The actual delivered free air (CFM FA) is the CFM displacement, multiplied by efficiency.

Whenever possible, use actual ratings.

Equipment Air Pressure Range	Type of Device	Avg Free Air Consumption (CFM)
70-100	Air Filter Cleaner	3.0
70-100	Body Polisher*	2.0
70-100	Body Sander*	6.0
70-100	Brake Tester	3.6
70-100	Carbon Remover*	3.0
120-150	Car Rocker*	6.75
70-100	Car Washer*	2.5
70-100	Dusting Gun (blow gun)	2.5
120-150	Grease Gun (high pressure)*	3.0
70-100	1/8" & 1/2" Impact Wrench	2-7
70-100	3/4" Impact Wrench	10-20
70-100	1-1/4" Impact Wrench	20-40
70-100	Pannel Cutter	13
70-90	1/4" Screw Driver	9
70-90	1/4" Drill	7-13
70-90	2" Medium Duty Grinder*	10-20
70-90	6" Medium Duty Grinder	20-40
70-90	Laundry Press	2
70-90	Dry Cleaning Press	3
70-90	Sander	7-10
Hammers
70-100	Air Hammer - Light*	4-8
70-100	Air Hammer - Medium*	10-20
70-100	Fender Hammer*	8.75
70-100	Cement Breaker*	75
Lifts
70-100	Hoist (one ton)	1.0
145-175	Hydraulic Lift*	6.25
120-150	Pneumatic Garage Door	2.0
70-100	Radiator Tester	1.0
70-100	Spark Plug Cleaner	5.0
70-100	Spark Plug Tester	0.5
Spray Guns
70-100	Engine Cleaner*	5.0
70-100	Paint Spray Gun (production)*	8.5
70-100	Paint Spray Gun (touch-up)*	2.25
70-100	Paint Spray Gun (undercoating)*	19.0
70-100	Spring Oiler	3.75
70-100	Trans. & Diff. Flusher	3.0
Tire Tools
70-100	Tire Hammer	18
70-100	Bead Breaker	18
120-150	Rim Stripper	5.0
120-150	Tire Changer	1.0
120-150	Tire Inflation Line	1.5
120-150	Tire Spreader	1.0
120-150	Vacuum Cleaner*	6.5

Step #4: Using Compressed Air for Blowing

Pressure (psi)	CFM of Free Air for Orifice Diameter
	1/32"	1/16"	3/32"	1/8"	1/4"
50	0.91	3.64	8.2	14.5	58.2
70	1.19	4.76	10.7	19.0	76.0
80	1.33	5.32	11.9	21.2	85.0
100	1.61	6.45	14.5	25.8	103.0
120	1.90	7.58	17.0	30.2	121.0
150	2.33	9.20	20.7	36.7	147.0

Step #5: Air Storage Tank Capacity

Capacity in Gallons	10	20	30	60	80
Capacity in Cubic Feet	1.25	2.7	4.0	8.0	10.8
Cubic Feet Free Air @ 100 psi	10.5	21.0	31.0	62.1	84.0
Cubic Feet Free Air @ 150 psi	15.0	30.2	45.0	89.5	120.8
Note: 1 cubic foot = 7.48 gallons

Step #6: Motor Loads / Recommended Wiring

Single Phase
115V HP	1/2	3/4	1	1-1/2	2	3
Motor Amp	10	14	16	19	22	34
Line Fuse	30	45	50	60	80	110
Min. Wire Size	14	14	12	10	8	6
230V HP	1/2	3/4	1	1-1/2	2	3	5
Motor Amp	5	7	8	9	12	17	23
Line Fuse	15	25	25	30	40	60	70
Min. Wire Size	14	14	14	14	12	10	8

Three Phase
230V HP	1	1-1/2	2	3	5	7-1/2	10	15	20	25
Motor Amp	4	5	6	9	15	22	27	40	52	64
Line Fuse	15	15	20	30	45	70	90	125	175	200
Min. Wire Size	14	14	14	14	12	10	8	6	4	3
460V HP	1	1-1/2	2	3	5	7-1/2	10	15	20	25
Motor Amp	2	2.5	3	4.5	7.5	11	14	20	26	32
Line Fuse	15	15	15	15	25	35	45	60	80	100
Min. Wire Size	14	14	14	14	14	14	12	10	8	8

Step #7: Compressed Air Lines
(based on approx. 100 psi initial pressure)

Air pressure loss, psi, per 100 ft. length:

Volumn Air CFM 5 10 20 30 50 70 100

Pipe 1/2" 3/4" 1" 1-1/4"         0.38 0.09 0.03 0.01 1.42 0.34 0.10 0.03 3.13 0.74 0.23 0.06 8.65 2.00 0.60 0.15 3.85 1.12 0.28 7.80 2.21 0.55

Air Hose (ID) 5/16" 3/8" 1/2" 3/4" 3.0 0.5 8.0 2.0 30.0 10.0 63.0 31.0 4.6 70.0 16.0 36.0 4.0 7.5